Abrasive blasting devices operate on the physical property that gas at a higher pressure flows towards and into gas at lower pressure. When abrasive powder is mixed with gas at higher pressure, the gas carries the abrasive powder as the gas accelerates and flows to the lower pressure. As the gas and abrasive powder blast the target material at high speed, the impact of the particles removes layers of the target material.
This process of material removal is commonly known as etching and also as sandblasting. As the rate of the target material removal increases, the etching process can be utilized for drilling and cutting. More specifically, the aggressiveness of the particulate impact-speed and frequency determine the rate of material removal, and thus whether an abrasive blasting device is useful for polishing, etching, or drilling. Particulate impact-speed and impact-frequency are adjusted by variation of the gas flow rate and gas-to-particulate mixture ratio through perturbation of the abrasive material.
In dentistry this technology is known as micro-abrasion and is used to achieve a variety of goals—such as to remove foreign material or to dull a shiny surface, roughen or etch the surface to enhance bonding quality and to remove decay by drilling and cutting tooth structure. To facilitate such procedures, it is of supreme importance to select the precise quantity of abrasive powder introduced into the gas stream. As the rate of powder delivery or concentration of abrasive in the gas stream is increased, the greater the cutting rate of the device. So for procedures that only require light etching, a reduced amount of abrasive particles must be present in the gas stream, while for drilling and cutting procedures elevated quantities of abrasive particles in the gas stream provide for most efficient operation. Such delicate procedures performed intra-oral require instantaneous response and precise control over the flow of the particle stream to prevent damage due to over-etching.
Once the dentist has selected the abrasive concentration, it is of equal importance for the device to maintain a consistent powder delivery rate. Significant damage can be caused by an unexpected increase or decrease in particulate concentration during an intra-oral procedure. Over-etching of a tooth surface due to an increase in abrasion rate leads to permanent tooth damage. Under etching of a tooth surface due to a decrease in abrasion rate may cause weak bonding and/or trapped bacteria under the sealants.
Most air abrasion devices provide complex mechanisms to allow adjustment and assure the consistency of the abrasive concentration introduced into the air stream. Deardon et al. U.S. Pat. No. 6,083,001 discloses a dental air abrasion system in which the flow of the particles is electronically controlled by pressure differentials. Rainey U.S. Pat. No. 6,093,021 discloses an automated control system which utilizes a gas stream mounted particulate sensor to regulate fluid flow rates into and around the ultrasonically agitated mixing chamber in order to accurately maintain the abrasive concentration in the air stream.
Simple self-contained air abrasion devices—such as by Stark et al., U.S. Pat. No. 4,475,370, Hertz, U.S. Pat. Nos. 5,839,946 and 6,287,180, Hertz PCT application 96/11696 filed on Jul. 15, 1996, Hertz et al. U.S. Pat. No. 6,293,856, Schur et al. U.S. Pat. No. 6,004,191, Trafton et al. U.S. Pat. No. 6,354,924, and Groman U.S. Pat. No. 6,398,628 and U.S. Pat. No. 6,347,984—rely on the air stream to perturb the abrasive and generate the mixing action. The etching rate of these devices is adjustable only by the inlet pressure for a given nozzle aperture, since it is the flow rate of the air stream that generates the powder perturbation. The greater the inlet pressure, the greater the gas flow rate through the device and thus the increase in abrasive delivery rate. Conversely low powder delivery rates are attained by reduction of the inlet gas pressure.
For these devices however, as the inlet pressure is reduced the flow rate through the device is reduces and so does the impact speed of the abrasive powder. The reduction in impact speed leads to a reduction in operational efficiency, since the lower impact speed of the particulates requires more operational time and a greater quantity of powder material to accomplish a task. Therefore, operation of light etching is achieved only at lower operational pressures and therefore lower particulate velocities. Additionally, since these self-contained devices utilize the inlet pressure for concentration selection, they only perform at their optimum efficiency at the maximum inlet pressure.
Additionally, self-contained air abrasion devices contain a preset amount of powder material within the mixing chamber. The operational time of the device is the time period during which powder is contained within the mixing chamber. In practice, the operational time of the device is defined to terminate when the powder delivery rate reaches such a low level that the device is not able to perform useful etching.
Self-contained air abrasion devices experience a change in the powder delivery rate as the contained abrasive is depleted from the mixing chamber. As the abrasive discharges from the mixing chamber, the location of the delivery conduit outlet changes with respect to the remaining powder material. When the prior art device shown in FIG. 1A initiates operation, the delivery conduit outlet is submerged within the abrasive material. But as the quantity of abrasive is reduced during operation, the discharge conduit outlet becomes more distant from the abrasive material, as shown in FIG. 1B. As the distance between the outlet and the powder material increases, less powder material is perturbed by the inlet air stream and thus less material introduced into the air steam.
FIG. 1C depicts the powder delivery rate for this prior art device over its operational time for a constant pressure air supply. Over the device's operational time the powder delivery rate decreases as the powder depletes from the device and the distance between the abrasive powder and the delivery conduit outlet increases. This leads to a decrease in perturbation of the powder material as the gas flow is further from the powder material. The device's operational time terminates when it no longer has a powder delivery rate capable of performing useful etching of the target material. FIG. 1C shows that this device has a large variation in powder delivery rate consistency. At the initial operational state powder delivery occurs at high rate and then rapidly decreases as the mixing chamber depletes of powder material.
The length of the delivery conduit is also a significant factor in the powder delivery rate. FIG. 2 depicts two configurations of prior art devices with fixed length delivery conduits. FIG. 2A illustrates a prior art device with a long delivery conduit that reaches deep into the powder material. Referring to FIG. 2C, this prior art device has a high powder delivery rate that rapidly depletes the quantity of powder material in the mixing chamber. The rapid depletion leads to a short operational time and a high variability in powder delivery rate. FIG. 2B illustrates a prior art device with a short delivery conduit that may be initially submerged in the powder material. Such a prior art device has a lower powder delivery rate that slowly depletes the quantity of powder material in the mixing chamber. The slow depletion leads to a longer operational time and a slower variability in powder delivery rate. As FIG. 2C illustrates, a short delivery conduit configuration provides lower capability to generate high powder perturbation and therefore always yields a lower powder delivery rate. Since the powder delivery rate is reduced, the operational time of the device is longer and the change of powder delivery rate is reduced, however, it is not capable of creating high powder delivery rates.
Powder perturbation rate is also affected by the diameter of the delivery conduit or its outlet aperture. Delivery conduit outlets with small apertures lead to high air velocities into the mixing chamber, while delivery conduit outlets with large apertures lead to low air velocities into the mixing chamber. FIG. 3 depicts two configurations of prior art devices with small and large delivery conduit diameters. FIG. 3A depicts a prior art device with a small diameter delivery conduit. FIG. 3C illustrates that a delivery conduit outlet with small aperture has a high powder delivery rate that rapidly depletes the quantity of powder material in the mixing chamber. The rapid depletion leads to a short operational time and a high variability in powder delivery rate. FIG. 3B depicts a prior art device with a large diameter delivery conduit. FIG. 3C illustrates that a delivery conduit outlet with a large aperture has a lower powder delivery rate that slowly depletes the quantity of powder material in the mixing chamber. The slow depletion leads to a longer operational time and a slower variability in powder delivery rate. FIG. 3C shows that prior art devices with fixed delivery conduit diameters have large variations in powder perturbation consistency over the operational time of the device.
Hence, the length (position) or diameter (aperture) of the delivery conduit(s) are both “perturbation-determining characteristics (or features)” which can be used, alone or in combination with one another, to control powder delivery rate.
As shown in FIGS. 1, 2, and 3, perturbation rates of prior art simple air abrasion devices are dependent on the 1) amount of material depleted from the device, 2) length of the delivery conduit, 3) diameter and apertures of the delivery conduits. Prior art simple air abrasion devices compensate for this deficiency by requiring the user to constantly adjust the inlet gas pressure. Adjustment of the inlet supply gas pressure controls the flow rates through the device and therefore the inlet air velocities. Higher inlet air velocities allow for increase in material perturbation at greater distances between the delivery conduit outlet and the powder material. Therefore, over the operational time of the instrument, the user must adjust the input pressure to compensate for the decline in powder delivery rate. Additionally, since the inlet pressure is utilized to control the powder delivery rate, the user must also modify the input pressure to select the perturbation intensity for the required procedure.
Consequently, since low perturbation rates are only achieved at lower supply pressures, the user must operate at lower pressures for procedures that require reduced powder delivery rates. As the powder delivery rate of the device is reduced, the efficiency of the device decreases since the abrasive powder is delivered at lower nozzle velocities.